Redmine

When receiving an email notification from Redmine, the inline image is a red cross.
The link seems to be correct, but I assume the mail client is not authorized (not authenticated) to see the file.

I suppose this could be fixed by changing some privileges on for example the anonymous role or so on... but I cannot find the correct settings.

The link works fine when I have been authenticated on Redmine, but not from within a not authenticated browser.

Could anyone give me some clues on what privileges to check?

Thank you